1. Field of the Invention
The present invention relates to a power supply capable of responding to a variation in load, and an image forming apparatus provided with the power supply.
2. Description of the Related Art
There has conventionally been known a power supply, as a configuration for applying bipolar voltage (hereinafter sometimes referred to as bias) which has a positive polarity and a negative polarity, in which a power supply for outputting a positive polarity and a power supply for outputting a negative polarity are connected in series to selectively output voltage having a positive polarity or voltage having a negative polarity. One example of a configuration of a conventional power supply for outputting voltage having a positive polarity and a negative polarity will briefly be described with reference to FIG. 8. As illustrated in FIG. 8, a negative bias generation circuit 1001 and a positive bias generation circuit 1003 are connected in series, and each output terminal is connected to a load 1005. When a transformer 1002 is driven in this configuration (a transformer 1004 is not driven), a negative bias is applied to the load, and when the transformer 1004 is driven (the transformer 1002 is not driven), a positive bias is applied to the load. The configuration of the power supply described above is discussed in Japanese Patent Application Laid-Open No. 6-318117.
However, in the configuration of the conventional power supply, the biases of both polarities are selectively obtained from the positive bias generation circuit and the negative bias generation circuit, each of which are connected with the load, thereby the circuit size becomes large. More specifically, this configuration prevents a power supply circuit from reducing its size, which prevents reduction of a size of a substrate or an apparatus. Further, this configuration needs to have another control signal for changing the positive bias and the negative bias in addition to a control signal for controlling a value of a target voltage output from the power supply. When there are a plurality of loads that need the positive and negative biases, the number of the control signal might be increased.